1. Field of the Invention
The invention relates to a burner system including a pre-mixing burner with an integrated diffusion burner for gas turbine combustion chambers.
2. Description of the Prior Art
The combustion of gaseous or liquid fuels in gas turbine combustion chambers produces as pollutants in the gas, among other things, the nitrogen oxides NO and NO.sub.2 which together are designated as NO.sub.x. The NO.sub.x -content in the exhaust gas is to be kept as low as possible; in the USA, for instance, a limit of 75 ppm at 15% by volume O.sub.2 must not be exceeded according to environment protection regulations. Complying with these regulations presents great difficulties particularly if conventional diffusion burners are used in the gas turbine combustion chambers. In such diffusion burners the fuel is introduced directly via a fuel nozzle into the combustion zone and is mixed there with the combustion air, where the combustion is controlled by turbulent diffusion processes between the fuel, the combustion air and the exhaust gas products. A relatively high NO.sub.x -content in the exhaust gas is obtained due to large temperature differences with high temperature peaks produced by local areas of different mixing ratios of fuel and combustion air. German Published Non-Prosecuted Application DE-OS No. 29 50 535 relating to gas turbine combustion chamber discloses so-called pre-mixing and pre-evaporation of the liquid fuel fed-in through fuel nozzles and/or the pre-mixing of the gaseous fuel fed-in through fuel nozzles with the combustion air takes place with a large excess air number. Pre-mixing avoids local spots or areas of different mixing ratios of fuel and combustion air in the combustion. On the other hand, such premixing burners have a substantially smaller operating range than conventional diffusion burners, i.e., the mixing ratios of the fuel and the combustion air must be kept within relatively narrow limits. If the fuel content is too high, too much NO.sub.x is formed, and if there is too little fuel, the combustion is extinguished. The fuel lines of the individual pre-mixing burners are therefore equipped with valves so that, as a function of the load of the gas turbine, only as many pre-mixing burners are supplied with fuel, as can be kept within the narrow limits of the mixing ratios for the operation. In addition, several conventional diffusion burners are provided which serve for starting up the gas turbine and are subsequently switched off again.
German Published Non-Prosecuted Application DE-OS No. 26 13 589 discloses hybrid burners for gas turbine combustion chambers which consist of pre-mixing burners with integrated diffusion burners. The pre-mixing burner has a pre-mixing chamber which is confined at the down-stream end by a flame retention baffle and into which a main fuel nozzle and a feed device for combustion air open. The diffusion burner has a pilot fuel nozzle which is arranged in the central zone of the flame retention baffle, where the combustion air for the operation of the diffusion burner is fed-in in part through the pre-mixing chamber of the pre-mixing burner and in part through holes arranged in the flame retention baffle of the gas turbine combustion chamber. In addition, a fuel control device is provided which controls the total amount and the partial amounts of the fuel fed to the main fuel nozzle and the pilot fuel nozzle as a function of the load of the gas turbine in such that, until the idling speed or a small partial load is reached, the diffusion burner can be operated exclusively, and thereafter, the diffusion burner and the pre-mixing burner operate together. Such hybrid burners therefore have the advantage that separate starting burners designed as diffusion burners can be dispensed with in a space-saving, compact design. In addition, the flame of the diffusion burner supports the combustion of the mixture of fuel and combution air generated in the pre-mixing chamber, i.e., the flame of the pre-mixing burner can be ignited reliably with mixing ratios extremely lean in view of low NO.sub.x development. The hybrid burner can, therefore, be operated over a wide load range with low NO.sub.x -content in the exhaust gas. However, an undesirable rise of the NO.sub.x -development in the upper load range and especially at the load limit is noted.